1. Field of the Invention
The invention relates to a process for applying liquid, pasty or foamy substances to both sides of webs of material, particularly swellable webs of material, by means of applicator rolls disposed one behind the other in the direction of movement of the web of material, followed by drying in a suspension drier.
2. Discussion of Prior Art
There is no problem in applying solvent-containing substances to webs of material, such as papers or nonwovens, since the solvent influences the behaviour of the web of material. In dependence of the kind of material, the web of material may stretch, shrink or fold as it runs over deflector or applicator rolls. It is true that in order to avoid folding, stretching rolls have been disposed downstream of the first applicator roll and/or the web of material has been levelled by intermediate drying. However, stretching rolls can be used to a limited extend, and may even be out of the question if the web has become sticky due to being impregnated with the substance to be applied, for example, a resin.
Another disadvantage of conventional processes is that the substances to be applied on both sides of the web have different periods of action available until drying, the result being loss of quality.
These disadvantages exist, for example, in a known apparatus for application on both sides, wherein the web of material is deflected around each of two deflector rolls and is therefore guided in S-shape. Between the first and second deflector rolls, an applicator roll applies the substance to one side of the web from below, the other substance being applied also from below to the other side of the web downstream of the second deflector roll. Due to the long path between the two places of application and the deflector roll the aforedescribed disadvantages may occur, more particularly with swellable and porous webs of material. Moreover, with such an arrangement of deflector and applicator rolls it is difficult to draw in the web of material.
Another difficulty arises in conventional processes if different kinds of substances are to be applied to the two sides of the web. In such cases there is a risk that the substances will become mixed with one another if the time elapsing until drying is too long.
Lastly, practical requirements demand that the substances shall be applicable not only uniformly, but precisely metered.